MANILA, Sept 11 (Reuters) - The Philippines plans a cut of 75 percent in spending next year on drug rehabilitation facilities, while at the same time seeking a massive hike in funding for a war on drugs that has killed thousands, fuelling concerns among lawmakers.

The government has rejected criticism that it lacked the commitment to rehabilitate drug users, saying it has attracted financing and is building treatment facilities, but had underestimated the scale of addiction.

More than 3,800 people, most of them drugs suspects, have died in police operations in the drug war unleashed by President Rodrigo Duterte in July last year.

Police deny they were executing suspects, saying those killed had violently resisted arrest.

Senator Ralph Recto, who has questioned the government's anti-drugs budget, said he would scrutinise its proposal to cut expenditure on drug rehabilitation centres by 2.3 billion pesos ($45.23 million), compared to this year.

The government has asked Congress for an increase of more than 40 times in next year's police budget for anti-drugs operations next year.

"I will discuss these issues when the budget is formally presented," Recto told Reuters in a text message.

The lack of rehabilitation centres would cripple the "declared government policy" to wean substance abusers off drugs, Recto said in a statement over the weekend.

In August, Health Secretary Paulyn Ubial said eight drug rehabilitation centres would be built across the Philippines, funded by private firms, including conglomerate San Miguel Corp and property firm Megaworld.

Last year, Manila opened what it called a "mega" drug rehabilitation facility to treat up to 10,000 patients and funded by a Chinese tycoon.

The Department of Health submitted a budget of 759.6 million pesos for state-managed rehabilitation facilities under the government's proposed 2018 spending plan, significantly less than this year's budget of 3.08 billion pesos.

Methamphetamine use for a year or more would shrink the brain of a person, Duterte said in a speech in August last year, adding, "Therefore he is no longer viable for rehabilitation."

He estimated there are already more than 3 million Philippine drug users in a country of more than 100 million people.

The government's 13 drug abuse treatment and rehab facilities treated 14,733 out-patients in 2016, up more than three times from the previous year, and close to 30,000 in- patients, up four percent over 2015, the health department says.

Duterte has said the drugs war will continue and would be "unremitting as it will be unrelenting". ($1=50.8540 Philippine pesos) (Reporting by Karen Lema; Editing by Clarence Fernandez)